Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for at least fifty years and have led to the relatively recent market introduction of a number of superior polyalpha-olefin synthetic lubricants, primarily based on the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index, while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These new synthetic lubricants lower friction and hence increase mechanical efficiency across the full spectrum of mechanical loads from worm gears to traction drives and do so over a wider range of operating conditions than mineral oil lubricants.
The chemical focus of the research effort in synthetic lubricants has been on the polymerization of 1-alkenes. Well known structure/property relationships for high polymers as contained in the various disciplines of polymer chemistry have pointed the way to 1-alkenes as a fruitful field of investigation for the synthesis of oligomers with the structure thought to be needed to confer improved lubricant properties thereon. Due largely to studies on the polymerization of propene and vinyl monomers, the mechanism of the polymerization of 1-alkene and the effect of that mechanism on polymer structure is reasonably well understood, providing a strong resource for targeting on potentially useful oligomerization methods and oligomer structures. Building on that resource, in the prior art oligomers of 1-alkenes from C.sub.6 to C.sub.20 have been prepared with commercially useful synthetic lubricants from 1-decene oligomerization yielding a distinctly superior lubricant product via either cationic or Ziegler catalyzed polymerization. Particularly useful 1-alkene oligomers have been found to have a relatively low ratio of methyl to methylene groups in the oligomer. The ratio is called the branch ratio and is calculated from infrared data as discussed in "Standard Hydrocarbons of High Molecular Weight," Analytical Chemistry, Vol. 25, No. 10, p. 1466 (1953) Viscosity index has been found to increase with lower branch ratio. Heretofore, oligomeric liquid lubricants exhibiting very low branch ratios have not been synthesized from 1-alkenes. For instance, oligomers prepared from 1-decene by either cationic polymerization or Ziegler catalyst polymerization have branch ratios of greater than 0.20. Shubkin, Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 15-19, provides an explanation for the apparently limiting value for branch ratio based on a cationic polymerization reaction mechanism involving rearrangement to produce branching. Other explanations suggest isomerization of the olefinic group in the one position to produce an internal olefin as the cause for branching. Whether by rearrangement, isomerization or a yet to be elucidated mechanism, it is clear that in the art of 1-alkene oligomerization to produce synthetic lubricants as practiced to-date excessive branching occurs and constrains the limits of achievable lubricant properties, particularly with respect to viscosity index. Obviously, increased branching increases the number of isomers in the oligomer mixture, orienting the composition away from the structure which would be preferred from a consideration of the theoretical concepts discussed above.
Theoretically, the oligomerization of 1-decene, for example, to lubricant oligomers in the C.sub.30 and C.sub.40 range can result in a very large number of structural isomers. Henze and Blair, J.A.C.S. 54, 1538, calculate over 60.times.10.sup.12 isomers for C.sub.30 -C.sub.40. Discovering exactly those isomers, and the associated oligomerization process, that produce a preferred and superior synthetic lubricant meeting the specification requirements of wide-temperature fluidity while maintaining low pour point represents a prodigious challenge to the workers in the field. Brennan, Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 2-6, cites 1-decene trimer as an example of a structure compatible with structures associated with superior low temperature fluidity wherein the concentration of atoms is very close to the center of a chain of carbon atoms. Also described therein is the apparent dependency of the oligomer on the oligomerization process, i.e., cationic polymerization or Ziegler-type catalyst, known and practiced in the art.
U.S. Pat. No. 4,827,064 to Wu discloses high viscosity index synthetic lubricant compositions. These materials exhibit a low branch ratio, high viscosity index and low pour point. These compositions comprise C.sub.30 -C.sub.1300 hydrocarbons, said compositions having a branch ratio of less than 0.19; weight average molecular weight between 300 and 45,000; number average molecular weight between 300 and 18,000; molecular weight distribution between 1 and 5 and pour point below -15.degree. C. These low branch ratios and pour points characterize the compositions of the invention, referred to herein as polyalpha-olefin or HVI-PAO, conferring upon the compositions especially high viscosity indices in comparison to commercially available polyalpha-olefin (PAO) synthetic lubricants. Such compositions have been prepared by oligomerization of alpha-olefins such as 1-decene under oligomerization conditions in contact with an amorphous silica-supported and reduced valence state metal oxide catalyst from Group VIB of the IUPAC Periodic Table, e.g., chromium oxide. U.S. Pat. No. 5,012,020 discloses the preparation of a similar HVI-PAO with viscosity at 100.degree. C. greater than 15,000 cs which can be used as a Newtonian lube blend.
U.S. Pat. No. 5,105,051 discloses the preparation of HVI-PAO olefin oligomers over a catalyst comprising mesoporous siliceous material, e.g. MCM-41, and reduced Group VIB metal, e.g. chromium, in form of its oxide.
The entire contents of these disclosures are incorporated herein by reference.
Despite the desirability of the product produced by the process of Wu et al., the amorphous silica catalysts used are difficult to provide in a highly active form. Moreover, the mechanical strength of the amorphous silica catalyst can be less than what is necessary to maintain catalyst integrity during numerous reaction and regeneration cycles.